The present application relates to a hermetic seal device, into which a polarization-maintaining optical fiber penetrates from one space to the other through a partition for achieving airtightness of the polarization-maintaining optical fiber, and a hermetic sealing partition therefor.
Means for inserting an optical fiber into a partition dividing a space requiring airtightness includes a method in that the partition is provided with a hole having a diameter similar to that of the optical fiber and the clearance between the optical fiber and the hole is filled with a sealing compound such as low melting glass and a resin. Specifically, an airtight sealing structure is disclosed in that an optical fiber is inserted into a metallic pipe having a small-diameter low melting-glass injection hole, and the low-melting glass is poured through the hole (see Japanese Unexamined Patent Application Publication No. H6-67038, [0002] [0011], for example).
According this Publication, by the strain due to differences in expansion coefficient of materials (optical fiber 5×10−7/K, low melting glass 60×10−7/K, and metallic pipe 180×10−7/K), polarization characteristics are deteriorated. Hence, according this Publication, by inserting a glass pipe between the optical fiber and the metallic pipe, the contact area between the optical fiber and the low melting glass is minimized so as to reduce the stress applied to the optical fiber.
Recently, the reduction in effect of the stress due to the difference in expansion coefficient has been demanded from industrial circles, and means for securely preventing polarization characteristics from being deteriorated is required. For example, when the polarization-maintaining optical fiber is incorporated in a displacement detection sensor, a spectrointerferometer, or a fixed-point detector, the required extinction ratio is 20 dB or more, and with increasing extinction ratio, the deterioration in accuracy is smaller relative to the deflection of the optical fiber.